Over the last decade, the optical fibers have been developed and installed as the backbone of interoffice networks for voice, video and data transmission. These are becoming important with growing and expanding telecommunication infrastructure. Their importance is further increasing because of their high bandwidth applicability. The higher bandwidth demand is further increasing exponentially with time because of rapid growth of information technology.
The capacity of light-wave communication systems has undergone enormous growth during the last decade. The growing bandwidth demand can be met by using a dense wavelength division multiplexing, hereinafter referred to as DWDM, approach with low dispersion fibers. The requirements of fiber have had to change to support these advances, especially the requirement for the amount and uniformity (slope) of chromatic dispersion across these wavelengths. The DWDM approach enhances the effective data rate of an optical fiber link by, increasing the number of wavelength channels within the wavelength band.
Conventionally, the multi-mode fibers at wavelength of about 850 nm were used, which were replaced by single mode fibers having zero dispersion wavelength near about 1310 nm in the wavelength region varying from about 1310 nm to about 1550 nm. The single mode or mono-mode optical fibers have greater bandwidth than that of the multimode fibers.
Therefore, the research has been directed towards the development of the single mode fibers, as these fibers were observed to have lower attenuation loss between the wavelength varying from about 1300 nm to about 1550 nm.
However, when single wavelength moved through 1550 nm window for lower attenuation loss, the single mode fibers were observed to have very high dispersion loss.
The major disadvantage of the single mode fibers with high dispersion loss at about 1550 nm was that it restricted higher bit rate transmission. This disadvantage of single mode fibers has been overcome by the improved single mode fibers, known as zero dispersion shifted optical fibers, which have zero dispersion [ZD] at a wavelength of about 1550 nm, that is even when the wavelength shifted to about 1550 nm.
The theoretical analysis reveals that a single mode fiber having low dispersion loss and low dispersion slope with higher effective area is most desirable for high capacity DWDM, as referred hereinabove, transmission. However, the dispersion shifted fibers used for long distance systems in the prior art have resulted in poor DWDM performance due to non-linear effects, for example, four wave-mixing, self phase modulation, cross phase modulation etc. caused by zero dispersion region within the DWDM window. The dispersion flattened fiber which specify the dispersion magnitude less than 2 ps/nm.km between 1.3 to 1.6 μm have zero dispersion region within the DWDM window. This result is strong four wave mixing, which prevents good DWDM performance.
Ideally the dispersion of an optical fiber should have a constant value over the entire wavelength-operating region. However, the dispersion of fibers varies with the wavelength as the refractive index varies with the wavelength. Their dispersion slope S0 quantifies this variability. The smaller the slope, the lesser the dispersion varies with the wavelength. Another advantage of the low dispersion and low dispersion slope fiber is that its small dispersion allows its minimum dispersion to be increased to better suppress the Four Wave Mixing non-linearity, while still keeping the fiber minimum dispersion small enough for the signals to travel to longer distances with minimum need for dispersion and dispersion slope compensation.
The prior art of the chromatic dispersion fiber has been illustrated in FIG. 1. It is a result of material and the waveguide dispersion. In the theoretical treatments of intramodal dispersion it is assumed, for simplicity, that the material and the waveguide dispersion can be calculated separately and then added to give the total dispersion of the mode. In reality these two mechanisms are intrinsically related, since the dispersive properties of the refractive index, which gives rise to material dispersion, also affect the waveguide dispersion. Material dispersion occurs because the index refraction varies as a function of the optical wavelength. On the other way waveguide dispersion is a function of the refractive index profile shape.
The parameters, like relationship of refractive index, and values of refractive index and radius of each part of the fibre, like centre core, cladding(s), ring core(s) and outer core, and the relationship between radius of such parts of the fibre, and number of cores and claddings decide the characteristic properties of thus obtained fiber and the applications of thus obtained fiber.
Therefore, the fibers known in the art are distinguished by way of their characteristic properties, which in-turn are decided by various parameters as stated herein above, i.e. by relationship of refractive index and radius of each part of the fibre, like centre core, cladding(s), ring core(s) and outer core, and the ranges [or values] of refractive index and radius of such parts of the fibre, and number of cores and claddings.
The fibers as known in the prior art either have low non-linearity but high bend loss or have low bend loss but less effective area or may have, higher non-linearity and higher bend loss or may have non-uniform chromatic dispersion over the third and fourth window or high dispersion slope, that is the fibre will not have desired characteristic properties and will sacrifice one of the property for achieving another property.
The increasing complexity of the demands on the fiber makes the designer to think further to re-design the refractive index profile [relationship between refractive index of each part of the fiber]. This requires thinking to have more complex designs. However, the complex designs are very sensitive to the manufacturing processes. The optical and material physics limits the combination of the above-said parameters, i.e. the relationship of refractive index and radius of each part of the fibre, like centre core, cladding(s), ring core(s) and outer core, and the ranges [or values] of refractive index and radius of such parts of the fibre, and number of cores and claddings, which are required to be achieved for obtaining a fiber having desired properties for desired applications.
It has been observed that the end product is the compromise, wherein each parameter is required to be selected in such a manner so as to have best possible combination of the parameters, wherein one parameter is achieved without adversely affecting performance of the critical attributes and system requirements. Insensitive system modeling is then required with each parameter to understand its impact.
It has been observed that the dispersion and dispersion slope varies with the wavelength and refractive index varies with the wavelength.
Therefore, in view of variation of dispersion and dispersion slope with the wavelength and variation of refractive index with the wavelength constant efforts are being made to develop optical fibers which have desired dispersion and dispersion slope and yet have such a refractive index profile and the configuration which is easy to be achieved and accordingly it is easy to fabricate the desired fiber which is suitable in as wider range of the wavelength as possible, that is to develop dispersion optimized optical fibers for wideband optical transmission.
One such optical fiber has been developed and disclosed in U.S. Pat. No. 6,879,764. The dispersion shifted fiber disclosed in this US patent has low dispersion slope between 1530 to 1565 nm (C-band) and 1565 to 1625 nm (L-band) transmissions and has been found suitable for transmission of higher bandwidth over longer distance with more uniform chromatic dispersion over the third and fourth window and has also been found to have optimized mode field diameter to achieve low bending induced loss at 1550 nm and at the more critical 1600 nm wavelength. This fiber comprises a centre core, two claddings, a ring core and the outer glass region, wherein first cladding is provided onto the outer periphery of the centre core, second cladding is provided onto outer periphery of the first cladding and the ring core is provided onto outer periphery of the second cladding, and the outer glass region surrounds the ring core as shown in FIG. 2. The configuration of this fiber is such that the refractive indices of centre core and the ring core are higher than that of the outer glass region and refractive indices of the claddings are lower than that of the outer glass region and the refractive indices are so selected that dispersion and chromatic dispersion slope are low and the bend loss is also low, but only during C- and L-band transmissions meaning thereby it is not suitable for S-band application. The another limitation of this fiber is that it has non-zero dispersion only at 1550 nm and not at 1460 nm, and therefore, it does have non-linearity problem, and can be used for only one channel of transmission and not for more than one channel of transmissions in the S-band region.
The another such optical fiber has been developed and disclosed in U.S. patent application Ser. No. 10/763,403. In accordance with one embodiment of this US application, the dispersion shifted fiber disclosed in this US application has higher spot area and comprises a center core region, one cladding region, a ring core region and an outer glass region, wherein the cladding is provided onto the outer periphery of the center core, and the ring core is provided onto the outer periphery of the cladding, and the outer glass region surrounds the ring core region as shown in FIG. 3A. The configuration of this fiber is such that the center core and the ring core have refractive indices higher than the outer glass region and the single cladding region has lower refractive index than the outer glass region. In accordance with second embodiment of this US application, the dispersion shifted fiber comprises a center core region, cladding region, a ring core region and an outer glass region, wherein the cladding region is divided into two cladding regions —inner cladding region and outer cladding region with ring core region being disposed therebetween so as to have inner cladding region onto the outer periphery of the center core, and the ring core region onto the outer periphery of the inner cladding region, and the outer cladding region onto the outer periphery of the ring core region, and the outer glass region surrounding the outer cladding region as shown in FIG. 3B. The configuration of this fiber is such that the center core and the ring core have refractive indices higher than the outer glass region and the cladding region has lower refractive index than the outer glass region, but the refractive index of both the cladding regions—inner cladding region and outer cladding region is selected to be same. The refractive indices of core, cladding and ring core regions of the fibers disclosed in this US application are so selected that the fibers thus obtained have low dispersion slope, low dispersion loss and the higher effective area, but only during C- and L-band transmissions meaning thereby the fibers of this US application also are not suitable for S-band application. The another limitation of these fibers is also same as of above fiber of first prior art, that is, these have non-zero dispersion only at 1550 nm and not at 1460 nm, and therefore, these do have non-linearity problem, and can be used for only one channel of transmission and not for more than one channel of transmissions in the S-band region.